1. Field of the Invention
The present invention relates to an MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which radio excites nuclear spins of an object magnetically with an RF (radio frequency) signal having the Larmor frequency and reconstructs an image based on an NMR (nuclear magnetic resonance) signal generated due to the excitation and, more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to image under a suppression method for suppressing a target signal from a material such as fat or silicone.
2. Description of Related Art
Magnetic resonance imaging is an imaging method which excites nuclear spins of an object set in a static magnetic field with an RF signal having the Larmor frequency magnetically and reconstructs an image based on an NMR signal generated due to the excitation.
In the field of magnetic resonance imaging, there is a fat saturation method to acquire signals while suppressing signals from fat (fat signals). Conventionally, fat saturation methods used widely in general include CHESS (chemical shift selective) method, SPIR (spectral presaturation with inversion recovery) method (called SPECIR as well) and STIR (short TI inversion recovery) method.
In fat saturation methods, the CHESS method is called a selective fat saturation method since the CHESS method is a method to suppress only fat signals frequency-selectively using 3.5 ppm difference in resonant frequencies between water protons and fat protons (see, for example, Japanese Patent Application (Laid-Open) No. 7-327960, Japanese Patent Application (Laid-Open) No. 9-182729 and Japanese Patent Application (Laid-Open) No. 11-299753).
FIG. 1 is a diagram explaining a method for suppressing fat signals under a conventional CHESS method.
In FIG. 1, the abscissa denotes a frequency and the ordinate denotes a signal intensity of NMR signal. In a frequency selective fat saturation method, a static magnetic field is made uniform by shimming prior to imaging. As shown in FIG. 1, respective protons of water and fat have 3.5 ppm difference in a resonant frequency and, therefore, a peak of frequencies of fat signals becomes sharp when the uniformity of a static magnetic field is obtained enough by shimming. Then, when a fat saturation RF pulse with a 90 degree flip angle (FA) and a frequency matching a resonant frequency of fat, i.e., a CHESS pulse is applied to tilt only longitudinal magnetization of fat protons by 90 degree frequency-selectively and, subsequently, magnetization of fat protons substantially disappears by using a spoiler pulse previous to data acquisition, fat signals can be suppressed. As described above, the CHESS method is effective in fat saturation on a region with high uniformity with regard to a static magnetic field.
The SPIR method is also a selective fat saturation method that uses a difference in resonance frequencies between water protons and fat protons (see, for example, Japanese Patent Application (Laid-Open) No. 2006-149583).
FIG. 2 is a diagram explaining a method for suppressing fat signals under a conventional SPIR method. FIG. 3 is a diagram showing a TI for applying a pulse for excitation after applying the SPIR pulse shown in FIG. 2.
In FIG. 2, the abscissa denotes a frequency and the ordinate denotes a signal intensity of MR signal. Further, in FIG. 3, the abscissa denotes an elapsed time after applying an SPIR pulse and the ordinate denotes a longitudinal magnetization z of a proton spin.
In the SPIR method, an SPIR pulse which is a frequency-selective inversion RF pulse having a frequency matching with a resonant frequency of fat signals is applied. A FA of an SPIR pulse is set from 90 degrees to 180 degrees. When an SPIR pulse is applied, the proton spins in fat magnetized by a static magnetic field tilt by the angle according to the FA and a longitudinal magnetization z of the proton spins in fat shows a minus value. Accordingly, the longitudinal magnetization z of proton spins increases with time to show a plus value by longitudinal relaxation (T1 relaxation). Then, with setting an inversion time (TI) to the timing which a longitudinal magnetization z in fat reaches the null point by T1 relaxation after applying an SPIR pulse, an RF pulse for excitation of water signals is applied. This allows only water signals to be excited selectively.
In the SPIR method, if an angle to tilt proton spins in fat, i.e., suppression effect is set largely like the case that a FA of an SPIR pulse is set to 180 degrees, for example, to suppress fat signals is easy even in a fat region having a resonant frequency band with a broad base. Thus, since a FA of an SPIR pulse is larger than a FA of 90 degree pulse, an SPIR pulse has a larger frequency band to be able to suppress fat signals than that of a normal CHESS pulse. In the SPIR method, since there is a dead time TI until the application timing of an RF pulse for excitation after to an application of an SPIR pulse, the waveform of an SPIR pulse can be better approximated to a rectangle.
In the SPIR method, though it is necessary to acquire data after a TI which is the time until an intensity of fat signal (a longitudinal magnetization z of a proton) becomes the null point, the TI can be also shortened by reducing a FA of an SPIR pulse. Generally, a TI is approximately from 150 ms to 180 ms in a 1.5 T MRI apparatus and a TI is approximately from 200 ms to 250 ms in a 3 T MRI apparatus.
On the other hand, the STIR method is a fat saturation method to use a difference in T1 relaxation times between fat and water signals and is a non-selective fat saturation method. Therefore, in the STIR method, shimming is unnecessary. In the STIR method, a region including a fat region and a water region is excited by a STIR pulse with a 180 degree FA. As shown in FIG. 2, the T1 relaxation time of water shown with the dotted line is longer than the T1 relaxation time of fat. Then, the timing which a longitudinal magnetization z in fat protons becomes the null point is set to the TI and an RF pulse for excitation is applied. Then, after a lapse of the TI, a longitudinal magnetization z of water protons does not reach the null point yet and only water protons can be excited selectively.
However, since a CHESS pulse is an RF pulse with a 90 degree FA, when uniformity of static magnetic fields in a water region and a fat region is not satisfactory, fat signals cannot be suppressed sufficiently.
FIG. 4 is a diagram showing an example case of insufficiently suppressed fat signals in case of performing fat-saturation under a conventional CHESS method.
In FIG. 4, the abscissa denotes a frequency and the ordinate denotes a signal intensity of NMR signal.
As shown in FIG. 4, when the uniformity of the static magnetic field does not become satisfactory even if shimming is performed, the base of a resonant frequency in fat becomes wide. Specifically, in a region showing a large susceptibility such as a breast and a jaw, there is a trend in which a band of fat signals extends in the frequency direction. Therefore, even if a CHESS pulse with a 90 degree FA is applied, component of fat signals remains as indicated by arrows. Thus, the CHESS method has a disadvantage that it is difficult to suppress fat signals extending in a wide frequency band sufficiently in the case of an uneven static magnetic field.
FIG. 5 is a tomographic image of an object obtained with a non-uniform static magnetic field under a conventional CHESS method.
As shown in FIG. 5, in the case of an uneven static magnetic field, it is recognized that fat signals are not suppressed sufficiently and a fat region is depicted on a tomographic image of an object.
In the conventional SPIR method, when a longitudinal magnetization of fat protons does not become the null point with the result that the adjustments of a TI and a FA of fat saturation pulse are not appropriate, unsuppressed fat signals remain. Note that whether or not fat signals show null changes depending on a T1 value of fat. Therefore, when it is assumed that a T1 of fat value and TI are fixed, adjusting for a FA of a fat saturation pulse allows fat signals to show null logically. However, practically, fat signals often remain by the factors such as non-uniformity of a magnetic field, non-uniformity of an RF power (FA) of a fat saturation pulse and a subtle difference in a T1 value of fat within a living body.
FIG. 6 is a diagram showing an example case of fat signals remaining due to an inadequately adjusted TI or FA of a fat-saturation pulse under a conventional SPIR method.
In FIG. 6, the abscissa denotes a frequency and the ordinate denotes a signal intensity of NMR signal. In the SPIR method, in the case that the adjustment of a TI and a power of a fat saturation pulse is incomplete or by the factors such as influence of non-uniformity of a magnetic field and an RF power (FA) of a fat saturation pulse and a subtle difference in a T1 value within a living body even if a TI and a power of a fat saturation pulse are adjusted, fat signals remain as shown in FIG. 6. Further, the SPIR method has the disadvantage to obtain sufficient fat saturation effect at only a region showing a high uniformity with regard to a static magnetic field like the CHESS method.
FIG. 7 is a tomographic image of an object obtained with an inadequately adjusted FA of a fat-saturation pulse under a conventional SPIR method.
FIG. 7 shows an image obtained by setting a TI to be the shortest and adjusting a FA of a fat saturation pulse so that fat signals show null. As shown in FIG. 7, the fat saturation effect by the conventional SPIR method is better than that by the CHESS method. However, it is recognized that a fat region is depicted on the tomographic image of an object without suppressing fat signals sufficiently by the factors described above even if a FA of a fat saturation pulse is adjusted.
Although shimming is unnecessary since the STIR method is not a fat saturation method to suppress fat signals frequency-selectively, intensities of water signals to be acquired become low due to acquisition of the water signals subsequently to a lapse of a TI. Therefore, in the STIR method, there are problems that the SNR (noise-to-signal ratio) decreases and a decrease of the SNR leads to extension of an imaging time.
As mentioned above, in a conventional frequency selective fat saturation method, when uniformity of a static magnetic field cannot be obtained satisfactorily due to the influence of the shape of an imaging part like the case of imaging a region such as a jaw part and a breast, it is difficult to suppress fat signals sufficiently. On the other hand, in a conventional non-frequency selective fat saturation method, the intensities of water signals to be acquired decrease and an imaging time becomes long.
In addition, in the conventional frequency selective fat saturation method, when a parenchymal part of an object is to be suppressed, plural excitation pulses are applied repeatedly under the condition that each frequency of excitation pulse is matched with a resonant frequency of water. However, in the case of a high-speed imaging, an application interval of an excitation pulse becomes long and signal recovery from an imaging target occurs due to the influence of a longitudinal magnetization. Consequently, the suppression effect in the frequency direction by an excitation pulse does not become constant and the suppression effect of signals cannot be obtained sufficiently depending on a set frequency of an excitation pulse.
That is, though the suppression effect of fat signals can be evaluated by a slice profile showing signal intensity to a frequency variation of an excitation pulse, the profile changes according to an application interval of an excitation pulse. For example, in the case that data acquisition is performed using a sequence of FFE (fast field echo) type under the segment k-space method, if the number of segments is increased, the profile showing the suppression effect of fat signals is improved since an application interval of an excitation pulse becomes short; on the contrary, if the number of segments is decreased, a satisfactory profile cannot be obtained since an application interval of an excitation pulse becomes long.
Note that the segmented k-space method is a data acquisition method which segments data in k-space into several areas (segmentalization) and retrieves data from every segment sequentially. Therefore, the number of segments is equivalent to a number of divisions of phase encoding (PE) in k-space.
FIG. 8 is a diagram showing a profile representing a simulation result of the effect by conventional fat-saturation methods.
In FIG. 8, the abscissa denotes a frequency shift amount (ppm) of an excitation pulse from a resonance frequency of fat and the ordinate denotes a relative signal intensity of an echo signal in case of setting the maximum values as 1.
As shown in FIG. 8, it is confirmable that the profile in the case that the number of segments is two has more deteriorated stability of fat saturation effect in the frequency direction than the profile in the case that the number of segments is sixty-four since an application interval of an excitation pulse in the case that the number of segments is two is longer than that in the case that the number of segments is sixty-four.
Therefore, adjusting a waveform of an excitation pulse is needed in order to improve a slice profile showing the fat saturation effect. For example, when an excitation pulse has a sinc waveform, a waveform control such as expansion and contraction of an excitation pulse wavelength and change of a side lobe shape are necessary.
Though signals from an arbitrary matter such as water and silicone as well as signals from fat can be also suppressed, there is a problem as described above in the case that signals from a matter except fat are suppressed as well.